1. Field of the Invention
The present invention relates to a light guide plate and a surface light source device incorporating the light guide plate, especially to a light guide plate configure to provide high light intensity at a center thereof.
2. Prior Art
Liquid crystal displays are thin, lightweight, and have low power consumption. Owing to these characteristics, liquid crystal displays have been widely used in the fields of communications and consumer products, in devices such as personal computers, liquid crystal TVs, image telephones, PDAs (personal digital assistants), etc. The liquid crystal display is a passive device, which must usually incorporate a surface light source device in order to provide images.
A conventional surface light source device 10 is illustrated in FIG. 9. The surface light source device 10 includes a light guide plate 1, a reflector 2 arranged on a bottom surface 11 of the light guide plate 1, a diffuser 3 arranged on an emitting surface 12 of the light guide plate 1, a light source 6 positioned adjacent to an incident surface 13 of the light guide plate 1, a reflector positioned behind the light source 6, and two prism sheets 4, 5 arranged on the diffuser 3. In operation, light beams coming from the light guide plate 1 are scattered by the diffuser 3, and uniformly emit from the diffuser 3. The light beams are then converged by the prism sheets 4, 5 to improve the intensity of illumination. However, the surface light source device 10 has numerous elements, which makes it unduly thick and costly. The light beams must pass through a plurality of interfaces before emitting from the prism sheet 5, which reduces the intensity of the emitted light.
FIG. 10 shows a surface light source device 20 as disclosed in China Patent No. CN99214211. The surface light source device 20 includes a light guide plate 21, a reflector 22, a light source 24, and a diffusing film 23. A plurality of terraced printing lines 261 is defined on an emitting surface 26 of the light guide plate 21, to enhance the light intensity thereof. However, light beams from the light source 24 are converged only along a direction of the printing lines 261. Therefore, the light beams cannot be utilized efficiently, and the light intensity of the surface light source device 20 is relatively poor.
In order to solve these problems, China Patent No. CN98207946 provides a light guide plate which comprises a prism array with single axis on an emitting surface, in order to increase the intensity of emitted light. Alternatively, a prism array with two axes can be defined on the emitting surface. However, a liquid crystal display generally needs a higher intensity of illumination in a center region of a display screen thereof, in order to satisfactorily illuminate a main visible range of the liquid crystal display. The light guide plate does not provide the required illumination.